Liquid encapsulants or molding compounds for semiconductor use require various properties as follows. Specifically, they should have a low viscosity for good coating workability. They should have a low coefficient of thermal expansion and undergo less warpage so as to have a coefficient of thermal expansion similar to that of the substrate to thereby prevent interfacial peeling (interfacial delamination) upon the application of thermal stress. They should contain small amounts of ionic impurities so as to avoid corrosion on IC chips. In addition, they should be stored satisfactorily stably (should have good storage stability (long shelf life)). As such liquid encapsulants for semiconductor use, there has been used a composition including a cycloaliphatic epoxy compound and a silica. The cycloaliphatic epoxy compound has a low viscosity and contains substantially no chlorine component because of using no epichlorohydrin as a starting material. The silica helps the composition to have a lower coefficient of thermal expansion.
The cycloaliphatic epoxy compound has good cationic curability. However, when the cycloaliphatic epoxy compound is used as a composition with a silica, the composition disadvantageously has poor storage stability because the cycloaliphatic epoxy compound, as having good cationic curability, undergoes a reaction with silanol groups in the silica. Specifically, the composition including the cycloaliphatic epoxy compound and the silica disadvantageously has poor storage stability, in which the composition is liable to have an increasing viscosity with time (namely, has poor viscosity stability).
As a possible solution to solve the poor storage stability, a technique of using a metal chelate catalyst in a composition has been proposed (see Patent Literature (PTL) 1). This technique, however, employs a metal, and the metal may probably adversely affect electric properties of a cured product obtained from the composition.